feat(app): secrets kept out of screenshots on macOS and Windows too

The reading of the full reading's fixes noted that macOS and Windows can
keep a window out of screen captures, which SecretScreen didn't ask.

PlatformScreenGuard now asks the app's window on Android, macOS and
Windows (asksOn), and both apps' runners answer app.halkyon/screen_guard,
counting what shows: macOS's MainFlutterWindow sets sharingType = .none,
Windows' FlutterWindow WDA_EXCLUDEFROMCAPTURE (Windows 10 2004 on;
before, WDA_MONITOR, black in captures). Neither runner was built here
(Linux): to be tried on each system.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/halkyon_learn_flutter/windows/runner/flutter_window.h

@@ -2,7 +2,9 @@
 #define RUNNER_FLUTTER_WINDOW_H_
 
 #include <flutter/dart_project.h>
+#include <flutter/encodable_value.h>
 #include <flutter/flutter_view_controller.h>
+#include <flutter/method_channel.h>
 
 #include <memory>
 
@@ -28,6 +30,17 @@ class FlutterWindow : public Win32Window {
 
   // The Flutter instance hosted by this window.
   std::unique_ptr<flutter::FlutterViewController> flutter_controller_;
+
+  // The channel halkyon_app's SecretScreen asks to keep the window out of
+  // screen captures while something secret shows (the recovery phrase,
+  // recovery codes, an authenticator's key), and how many show: counted, as
+  // several may at once.
+  std::unique_ptr<flutter::MethodChannel<flutter::EncodableValue>>
+      screen_guard_;
+  int secrets_shown_ = 0;
+
+  // Answers |screen_guard_|.
+  void GuardScreen();
 };
 
 #endif  // RUNNER_FLUTTER_WINDOW_H_